Today, we’ll move on to a new group of tumors of the CNS now: poorly-differentiated tumors.

These are tumors that arise from neuroectodermal cells, but are really primitive looking (they don’t have morphologic features of mature CNS cells). The most common of these poorly-differentiated tumors is medulloblastoma, a tumor that occurs most frequently in children.

Unlike many other CNS tumors, medulloblastoma occurs basically in one spot: the cerebellum. In children, it occurs in the midline, but in adults it sometimes occurs in lateral regions. If the tumor grows fast enough, it can occlude the flow of CSF, causing hydrocephalus. It’s usually well-circumscribed, and it has a propensity to extend up to the surface of the cerebellum and into the subarachnoid space (and hence the CSF), where it may spread to distant sites within the CNS. Sometimes, it metastasizes to the cauda equina (these are called “drop” metastases).

Histologically, medulloblastoma consists of sheets of tightly-packed, small, dark, anaplastic cells, which often have an elongated or crescentic shape. There are usually lots of mitoses. Sometimes the cells form little rosettes called Homer-Wright rosettes (which are also seen in neuroblastoma). They aren’t really true rosettes, if you want to be super accurate, because they don’t have a lumen (they just consist of cells surrounding a central region of neuropil). Compare these to the little rosettes (these are also technically pseudorosettes) we saw in ependymoma!

Without therapy, as you’d expect, prognosis is dismal. However, the silver lining is that this is one of a few malignancies that is happily, equisitely radiosensitive (another one is seminoma). With surgery and radiation, 5 year prognosis is around 75%.